During the past three decades, signifi cant advances have been made in understanding how environmental factors modify the bioavailability and the toxicity of metals such as copper in aquatic environments. Several of these advances have led to the development of guidelines to indirectly account for modifying factors, adjustment of criteria on a site-specifi c basis, and direct changes to the U.S. Environmental Protection Agency (U.S. EPA) freshwater quality criteria. To date, most of this effort has focused on freshwater systems, although similar modifying factors exist in marine environments as well. This paper focuses on one such modifying factor, dissolved organic carbon (DOC), and describes a method to aid in risk assessments or to refi ne the saltwater copper criteria on a site-specifi c basis. The relationship between DOC and toxicity of copper to the most sensitive saltwater genus in the U.S. EPA criteria database, Mytilus, is extensively analyzed. Dissolved copper 50% effective concentrations (EC50s) ).

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A model was recently published that relates the toxicity of copper to the most sensitive taxa in the U.S. Environmental Protection Agency's criteria database (Mytilus sp.) with dissolved organic carbon concentrations in saltwater. This model was developed for potential use in risk assessment and in the development of site-specific criteria (SSC) for copper in saltwater environments where Mytilus sp. is considered an appropriate indicator species. This manuscript presents the results of a field validation study of that model. Effective concentration 50% (EC(50)) values (n = 21) for seven sites were all predicted by the model within the previously established range of acceptability. Slopes and intercepts of the two data sets were not significantly different. Consequently, the data were pooled, and new equations were developed. Dissolved copper EC(50) values were highly correlated (r(2) = 0.76, n = 75, P < 0.0001) across a wide range of sample dissolved organic carbon (DOC) concentrations (0.3-12 mg C/L) and were explained by the equation EC(50) = 11.22 DOC(0.60). Two updated equations are proposed for consideration as a means of estimating site-specific final chronic criteria (FCC) and final acute criteria (FAC) for copper in marine and estuarine environments (copper FCC(DOC) = 3.59 DOC(0.60); copper FAC(DOC) = 5.61 DOC(0.60)).

The influence of dissolved organic carbon (DOC) and water composition on the toxicity of copper to juvenile freshwater mussels (fatmucket, Lampsilis siliquoidea) were evaluated in natural and reconstituted waters. Acute 96-h copper toxicity tests werec onducted at four nominal DOC concentrations (0, 2.5, 5, and 10 mg/L as carbon [C]) in dilutions of natural waters and in American Society for Testing and Materials (ASTM) reconstituted hard water. Toxicity tests also were conducted in ASTM soft, moderately hard, hard, and very hard reconstituted waters (nominal hardness 45-300 mg/L as CaCO₃). Three natural surface waters (9.5-11 mg/L DOC) were diluted to obtain a series of DOC concentrations with diluted well water, and an extract of natural organic matter and commercial humic acid was mixed with ASTM hard water to prepare a series of DOC concentrations for toxicity testing. Median effective concentrations (EC50s) for dissolved copper varied >40-fold (9.9 to >396 gg Cu/L) over all 21 treatments in various DOC waters. Within a particular type of DOC water, EC50s increased 5- to 12-fold across DOC concentrations of 0.3 to up to 11 mg C/L. However, EC50s increased by only a factor of 1.4 (21-30 gg Cu/L) in the four ASTM waters with wide range of water hardness (52-300 mg CaCO₃/L). Predictions from the biotic ligand model (BLM) for copper explained nearly 90% of the variability in EC50s. Nearly 70% of BLM-normalized EC50s for fatmucket tested in natural waters were below the final acute value used to derive the U.S. Environmental Protection Agency acute water quality criterion for copper, indicating that the criterion might not be protective of fatmucket and perhaps other mussel species.

Acute and chronic toxicity of copper (Cu) to a unionid mussel (Villosa iris) and a cladoceran (Ceriodaphnia dubia) were determined in water exposures at four concentrations of dissolved organic carbon (DOC; nominally 0.5, 2.5, 5, and 10 mg/L as carbon [C]). Test waters with DOC concentrations of 2.5 to 10 mg C/L were prepared by mixing a concentrate of natural organic matter (Suwannee River, GA, USA) in diluted well water (hardness 100 mg/L as CaCO(3) , pH 8.3, DOC 0.5 mg C/L). Acute median effect concentrations (EC50s) for dissolved Cu increased approximately fivefold (15-72 µg Cu/L) for mussel survival in 4-d exposures and increased about 11-fold (25-267 µg Cu/L) for cladoceran survival in 2-d exposures across DOC concentrations from 0.5 to 10 mg C/L. Similarly, chronic 20% effect concentrations (EC20s) for the mussel in 28-d exposures increased about fivefold (13-61 µg Cu/L for survival; 8.8-38 µg Cu/L for biomass), and the EC20s for the cladoceran in 7-d exposures increased approximately 17-fold (13-215 µg Cu/L) for survival or approximately fourfold (12-42 µg Cu/L) for reproduction across DOC concentrations from 0.5 to 10 mg C/L. The acute and chronic values for the mussel were less than or approximately equal to the values for the cladoceran. Predictions from the biotic ligand model (BLM) used to derive the U.S. Environmental Protection Agency's ambient water quality criteria (AWQC) for Cu explained more than 90% of the variation in the acute and chronic endpoints for the two species, with the exception of the EC20 for cladoceran reproduction (only 46% of variation explained). The BLM-normalized acute EC50s and chronic EC20s for the mussel and BLM-normalized chronic EC20s for the cladoceran in waters with DOC concentrations of 2.5 to 10 mg C/L were equal to or less than the final acute value and final chronic value in the BLM-based AWQC for Cu, respectively, indicating that the Cu AWQC might not adequately protect the mussel from acute and chronic exposure, and the cladoceran from chronic exposure.